feat: add support for stdin JSON input

[aduh95]

The goal is to remove the need to clone the repo in https://github.com/nodejs/node/blob/main/.github/workflows/notify-on-push.yml, and instead rely on push.commits like so:

- name: Validate commits
  run: echo "$COMMITS" | npx -q core-validate-commit -
  env:
    COMMITS: ${{ toJSON(github.event.commits) }}

[JakobJingleheimer]

Thanks for handling this :)

[JakobJingleheimer]

This seems likely to create bad DX. If it's not an array, just wrap it in an array.

[aduh95]

If there's a use-case for passing a non-array, we can consider it. Until then, it's probably more helpful to throw an error.